Daily Archives for October 3, 2017

Regional variations in sea surface temperature, related to seasons and El Niño, could be crucial for the survival of coral reefs, according to our new research. This suggests that we should be able to identify the reefs most at risk of mass bleaching, and those that are more likely to survive unscathed.

Climate warming is the major cause of stress for corals. The world just witnessed an event described as the “longest global coral die-off on record”, and scientists have been raising the alarm about coral bleaching for decades.

The first global-scale mass bleaching event happened in 1998, destroying 16% of the world coral reefs. Unless greenhouse emissions are drastically reduced, the question is no longer if coral bleaching will happen again, but when and how often?

To help protect coral reefs and their ecosystems, effective management and conservation strategies are crucial. Our research shows that understanding the relationship between natural variations of sea temperature and human-driven ocean warming will help us identify the areas that are most at risk, and also those that are best placed to provide safe haven.

A recurrent threat

Bleaching happens when sea temperatures are unusually high, causing the corals to expel the coloured algae that live within their tissues. Without these algae, corals are unable to reproduce or to build their skeletons properly, and can ultimately die.

The two most devastating global mass bleaching events on record – in 1998 and 2016 – were both triggered by El Niño. But when water temperatures drop back to normal, corals can often recover.

Certain types of coral can also acclimatise to rising sea temperatures. But as our planet warms, periods of bleaching risk will become more frequent and more severe. As a consequence, corals will have less and less time to recover between bleaching events.

A non-uniform response to warming

While the future of worldwide coral reefs looks dim, not all reefs will be at risk of recurrent bleaching at the same time. In particular, reefs located south of 15ºS (including the Great Barrier Reef, as well as islands in south Polynesia and Melanesia) are likely to be the last regions to be affected by harmful recurrent bleaching.

We used to think that Micronesia’s reefs would be among the first to die off, because the climate is warming faster there than in many other places. But our research, published today in Nature Climate Change, shows that the overall increase in temperature is not the only factor that affects coral bleaching response.

In fact, the key determinant of recurrent bleaching is the natural variability of ocean temperature. Under warming, temperature variations associated with seasons and climate processes like El Niño influence the pace of recurrent bleaching, and explain why some reefs will experience bleaching risk sooner than others in the future.

Different zones of the Pacific are likely to experience differing amounts of climate variability.Author providedDegrees of future bleaching risk for corals in the three main Pacific zones.Author provided

Our results suggest that El Niño events will continue to be the major drivers of mass bleaching events in the central Pacific. As average ocean temperatures rise, even mild El Niño events will have the potential to trigger widespread bleaching, meaning that these regions could face severe bleaching every three to five years within just a few decades. In contrast, only the strongest El Niño events will cause mass bleaching in the South Pacific.

In the future, the risk of recurrent bleaching will be more seasonally driven in the South Pacific. Once the global warming signal pushes summer temperatures to dangerously warm levels, the coral reefs will experience bleaching events every summers. In the western Pacific, the absence of natural variations of temperatures initially protects the coral reefs, but only a small warming increase can rapidly transition the coral reefs from a safe haven to a permanent bleaching situation.

One consequence is that, for future projections of coral bleaching risk, the global warming rate is important but the details of the regional warming are not so much. The absence of consensus about regional patterns of warming across climate models is therefore less of an obstacle than previously thought, because globally averaged warming provided by climate models combined with locally observed sea temperature variations will give us better projections anyway.

Understanding the regional differences can help reef managers identify the reef areas that are at high risk of recurring bleaching events, and which ones are potential temporary safe havens. This can buy us valuable time in the battle to protect the world’s corals.

Mercury pollution has a long legacy in the environment. Once released into the air, it can cycle between the atmosphere and ecosystems for years or even decades before ending up deep in the oceans or land.

To address the global and long-lasting mercury problem, a new United Nations treaty called the Minamata Convention on Mercury came into effect last month. The treaty commits participating countries to limit the release of mercury and monitor the impacts on the environment. Australia signed the Convention in 2013 and is now considering ratification.

Until now, we have only been able to guess how much mercury might be in the air over tropical Australia. Our new research, published in the journal Atmospheric Chemistry and Physics, shows that there is less mercury in the Australian tropics than in the northern hemisphere – but that polluted northern hemisphere air occasionally comes to us.

Small-scale gold mining is an economic mainstay for millions of people, but it releases mercury directly into the air and water sources.

Once released to the air, mercury can travel thousands of kilometres to end up in ecosystems far away from the original source.

Measuring mercury in the tropics

While the United Nations was gathering signatures for the Minamata Convention, we were busy measuring mercury at the Australian Tropical Atmospheric Research Station near Darwin. Our two years of measurements are the first in tropical Australia. They are also the only tropical mercury measurements anywhere in the Maritime Continent region covering southeast Asia, Indonesia, and northern Australia.

More surprising is the seasonal pattern in the data. There is more mercury in the air during the dry season than the wet season.

The Australian monsoon appears to be partly responsible for the seasonal change. The amount of mercury jumps up sharply at the start of the dry season when the winds shift from blowing over the ocean to blowing over the land.

In the dry season the air passes over the Australian continent before arriving at the site, while in the wet season the air usually comes from over the ocean to the west of Darwin.Howard et al., 2017 (modified)

But wind direction can’t explain the whole story. Mercury is likely being removed from the air by the intense rains that characterise the wet season. In other words, the lower mercury in the air during the wet season may mean more mercury is being deposited to the ocean and the land at this time of year. Unfortunately, there simply isn’t enough information from Australian ecosystems to know how this impacts local plants and wildlife.

Fires also play a role. Mercury previously absorbed by grasses and trees can be released back to the atmosphere when the vegetation burns. In our data, we see occasional large mercury spikes associated with dry season fires. As we move into a bushfire season predicted to be unusually severe, we may see even more of these spikes.

Air from the north

Although mercury levels were usually low in the wet season, on a few days each year the mercury jumped up dramatically.

To figure out where these spikes were coming from, we used two different models. These models combine our understanding of atmospheric physics with real observations of wind and other meteorological parameters.

Both models point to the same source: air transported from the north.

Australia is usually shielded from northern hemispheric air by a “chemical equator” that stops air from mixing. This barrier isn’t static – it moves north and south throughout the year as the position of the sun changes.

A few times a year, the chemical equator moves so far south that the top end of Australia actually falls within the atmospheric northern hemisphere. When this happens, polluted northern hemisphere air can flow directly to tropical Australia.

We observed 13 days when our measurement site near Darwin sampled more northern hemisphere air than southern hemisphere air. On each of these days, the amount of mercury in the air was much higher than on the days before or after.

Tracing the air backwards in time showed that the high-mercury air travelled over the Indonesian archipelago before arriving in Australia. We don’t yet know whether that mercury came from pollution, fires, or a mix of the two.

The highest mercury is observed when the air comes from the northern hemisphere.Howard et al., 2017 (modified)

A global solution

The cross-boundary influences on mercury that we have observed in northern Australia highlight the need for the type of multinational collaboration that the Minamata Convention will foster.

Our new data establish a baseline for monitoring the effectiveness of new actions taken under the Minamata Convention. With the first Conference of the Parties having taken place last week, hopefully it will only be a matter of time before we begin to see the benefit.

While both Ambae and Agung pose significant threats to local populations, they represent very different types of volcanoes.

In fact, the unique features of the Ambae volcano mean it presents immediate danger.

What’s special about the Ambae Volcano?

Ambae does not fit the stereotypical image of a volcano. Rather than being a steep-sided cone, it forms a low-angled mountain, reminiscent of shield lying flat on the earth.

Smoke billows from Vanuatu’s Manaro Voui volcano on Ambae island.

Instead of having a vertiginous vent filled by a lava lake (like its southern neighbour Ambrym), the summit contains a shallow depression featuring several water-filled lakes.

The largest of these, Lake Voui, is the current focus of volcanic activity, and looks unlike any lake you have seen before.

Volcanic gasses, including sulfur, chlorine and carbon dioxide, are discharged into the base of the lake. Not only do these make the lake highly acidic, but they typically give it a vibrant turquoise colour.

A volcanic lake on Mt Ruapehu in New Zealand, showing similar colour and chemistry to Lake Voui.C. Firth, Author provided

When the volcano last erupted in 2005, ash and lava built a cone in the centre of the lake, which eventually reached a height of around 50 metres above the lake surface.

As this happened, changing degrees of interaction between the lava, volcanic gases and the lake water caused fluctuations in its chemistry. This in turn changed the colour, which went from turquoise to battleship grey and then finally to a deep mahogany shade of red.

An annotated Landsat Image of Ambae Island taken on 19th July 2017. Look at the difference in colour of the two lakes on the summit of the volcano. Since this image was taken activity at the volcano has increased markedly.C. Firth, Author provided

Since then, the volcano has continued to emit huge volumes of gas, which have caused issues for local inhabitants over recent years, as they can lead to acid rain.

Acid rain can kill plants. This is a major issue on Ambae, as much of the population lives on staple crops such as banana and taro. These plants have large leaves that are particularly susceptible to acid rain.

Vegetation damaged by acid rain on neighbouring Ambrym volcano during 2014. The summit of Ambae can just be seen peeking out above the clouds in the far distance.C. Firth, Author provided

Over the past few weeks, gas emissions from Ambae have increased. Ash began to accompany the gas emissions around mid-September, suggesting that magma had reached the surface.

Satellite monitoring indicates that volcanic activity is continuing to escalate. Recent observations by New Zealand Air Force pilots noted lava blasting out of a crater in the centre of Lake Voui.

Is this part of the Ring of Fire?

Both Bali’s Agung and Ambae sit on the Pacific’s “ring of fire”, and the same tectonic forces are responsible for both volcanoes. However, closer links between the two volcanoes are very unlikely.

On any given day, there are generally 20-30 volcanoes erupting around the world (although normally these eruptions are on a smaller scale and are away from large populations, so they do not make the news).

Imagery taken during a New Zealand Defence Force aerial survey yesterday showed huge columns of smoke, ash and volcanic rocks billowing from the crater of Monaro volcano on Vanuatu’s Ambae Island.New Zealand Defence Force, CC BY

So how might the eruption at Ambae differ from Agung? The crater lake on Ambae offers particular hazards that might not be encountered elsewhere.

The first of these involves interaction between erupting lava and the lake water itself. The heat of the lava, which is likely to be 1,000-1,100℃, will rapidly turn lake water into steam, like dipping a hot frying pan into a sink of dishwater.

This scaled-up kitchen scenario can increase how explosive the eruption is, giving blasts from the volcano additional power. This may cause projectiles like lava bombs to go further, while also increasing the amount of ash produced.

A potentially more serious hazard may involve overflowing of the crater lake itself. If the eruption begins to displace water from the lake, it might trigger volcanic mudslides known as “lahars”, which would race down the volcano’s flanks, with the potential to inundate villages and gardens.

Local stories suggest villages on the island’s south coast were affected by lahars during the late 19th century, with significant loss of life.

Finally, there is a threat that activity may not be restricted to the volcano’s summit. The geological record indicates that magma has moved through fissures in the volcano’s flanks during previous eruptions, travelling laterally up to 20km from the centre of the volcano before erupting.

This means that rather than emerging on the sparsely inhabited summit of the volcano, lava may well erupt along the more densely populated coast. Such a scenario occurred in 1913 on the neighbouring volcano, Ambrym, where 21 people died.

The evacuation of the Ambae’s population will prevent such loss of life if this were to occur again.